The Swedish cruiser Gotland, under the command of Captain Agren, sighted the battleship Bismarck in the Kattegat on 20 May 1941 and immediately reported her position to Stockholm.
The sighting report was leaked to the British Naval Attaché, Captain Henry W. Denham who later alerted the Admiralty in London:

"Kattegat, today 20 May. At 1500, two large warships, escorted by three destroyers, five ships and ten or twelve planes, passed Marstrand to the northeast. 2058/20."